1. Field of the Invention
This invention relates to an anti-skid control apparatus for a vehicle braking system which can prevent the locking of the wheels.
2. Description of the Prior Art
Such an anti-skid control apparatus for a vehicle braking system is known that includes a fluid pressure control valve device arranged between a master cylinder and a wheel cylinder of a brake for the wheel, said fluid pressure control valve device receiving control signals of a control unit measuring the skid condition of the wheel to control the brake fluid pressure to the wheel cylinder; a hydraulic reservoir which, when the brake fluid pressure to the wheel cylinder is decreased with control of the fluid pressure control valve device, reserves the brake fluid discharged through the fluid pressure control valve device from the wheel cylinder; a pressure fluid supply conduit connecting the master cylinder with the fluid pressure control valve device; and a fluid pump for returning the brake fluid from the hydraulic reservoir into the pressure fluid supply conduit.
When the fluid pressure control valve device is provided for each of four wheels, and the fluid pressure of them are independently controlled, there is no problem on control operation. Or when the fluid pressure control valve device is provided for each of front wheels, and for both of rear wheels in common, there is no problem on control operation. In the latter case, the one common fluid pressure control valve device is controlled on the basis of the lower one of the speeds of the rear wheels.
However, in the above cases, three or four fluid pressure control valve devices are used. Accordingly, the whole anti-skid control apparatus is large-sized, and very heavy. Since the fluid pressure control valve device is expensive, it requires high cost.
For example, it is considered that the brake fluid pressures of the front wheels are controlled by the two fluid pressure control valve devices respectively in the diagonal or X-type conduit system, and the brake fluid pressures of the rear wheels are controlled in common with the front wheels. However, when the vehicle runs on the road, the right and left sides of which are considerably different in frictional coefficient from each other, there is the fear that the one rear wheel being diagonal to the one front wheel on the higher frictional coefficient side is locked. In that case, the steering of the vehicle becomes unstable, and that is very dangerous.
Further, it is considered that proportioning valves are provided for the rear wheels, respectively. However, the brake fluid pressures of the rear wheels increases in proportion to the input fluid pressures to the proportioning valves. The fear of locking cannot be avoided.
Accordingly, in order to provide an anti-skid control apparatus for a vehicle braking system which can be small-sized and light, and can avoid the fear of locking of rear wheels, this applicant previously proposed an anti-skid control apparatus for a vehicle braking system which includes; a fluid pressure control valve device arranged between a master cylinder and a wheel cylinder of a brake for the wheel, said fluid pressure control valve device receiving control signals of a control unit measuring the skid condition of the wheel to control the brake fluid pressure to said wheel cylinder; a hydraulic reservoir which, when the brake fluid pressure to said wheel cylinder is decreased with control of said fluid pressure control valve device, reserves the brake fluid discharged through said fluid pressure control valve device from said wheel cylinder; a pressure fluid supply conduit connecting said master cylinder with said fluid pressure control valve device; and a fluid pump for returning the brake fluid from said hydraulic reservoir into said pressure fluid supply conduit; said fluid pressure control valve device being provided for a pair of front wheels, respectively, a valve apparatus receiving fluid pressures of wheel cylinders of said front wheels being arranged between said pair of front wheels and a pair of rear wheels, and when any one of said fluid pressure control valve devices starts to control, at least the fluid pressure of the one of said rear wheels, being at the same side as the one of said front wheels, the fluid pressure of the wheel cylinder of which is lower, is controlled in accordance with the lower one of the fluid pressures of the wheel cylinders of said front wheels by said valve apparatus.
In the above-described anti-skid control apparatus, the control signals for the control unit are formed by judging the skid conductions of the respective front wheels. On the assumption that the front and rear wheels are provided with the types of the same kind, the braking forces are so distributed to the wheels that the front wheels tend sooner to lock than the rear wheels, when the vehicle is rapidly braked on the road which is uniform in frictional coefficient.
However, when the above assumption is not fulfilled, for example, when only the front wheels are provided with spike tyres or chains for running on a snow or ice road, and the rear wheels are provided with the normal tyres, the rear wheels tend sooner to lock than the front wheels. In the above anti-skid control apparatus, the brake fluid pressure is not controlled with the locking of the rear wheel. When the brake fluid pressure of the front wheel is controlled over the limit locking pressure of the rear wheel, the locking of the rear wheel is not released, and so the steering stability cannot be maintained.
Even in the case that the front and wheels are provided with the tyres of the same kind, the rear wheel may tend sooner to lock than the front wheel, when the frictional coefficient of the brake lining becomes excessively low due to thermal fade phenomenon in a front wheel brake apparatus and the limit lock pressure of the front wheel becomes excessively high, and particularly when the vehicle is rapidly braked on a higher road. When a proportioning valve is used, the fluid pressure of the rear wheel is lower than that of the front wheel. However, it increases in proportion to the fluid pressure of the front wheel, and reaches the limit lock pressure. The above described problem occurs.